1. Field of the Invention
The invention relates to an organic light emitting diode (OLED), and in particular to a host material in a light emitting layer of the OLED.
2. Description of the Related Art
Phosphate metal complexes have been utilized as dopants in OLED. Because of strong spin orbital coupling, the cyclometallated iridium complexes draw the most attention in this field. The triplet state lifetime of the iridium complexes is reduced by hybridization of the siglet and triplet states, thereby enhancing its quantum yield. In addition, the device efficiency can be improved by doping the phosphate materials into host materials. Therefore, the host materials have gradually become major research topics. In recent papers, the host materials for blue phosphate have revealed problems such as insufficient energy gap and low thermal stability due to small molecular weight. Please see Nature (London) 1998, 395, 151 by Baldo et al., J. Appl. Phys. 2001, 90, 4058 by Adachi et al., J. Org. Chem. 1976, 41, 3682 by Granoth et al., Appl. Phys. Lett. 2003, 83, 3818 by Holmes et al., and J. Org. Chem. 1976, 41, 3682 by Granoth et al.